Summary of Work: Unusual "fibroproliferative lesions", as well as A/B carcinomas with a variable connective tissue component, occurred in the lungs of female F344 rats after chronic inhalation of Cobalt Sulfate Hexahydrate. The characteristics of these lesions, both the connective tissue component and particularly the proliferation of their epithelial component, warrant investigation. The methodology is the same as that of investigations characterizing overall lung parenchymal alterations but centers on the tissue comprising and immediately surrounding the lesions. Counts of PCNA-reactive epithelium associated with these lesions would help clarify the possible progressive nature of the fibroproliferative lesions to a more neoplastic norphology.